The present invention relates generally to motion control, and, more particularly, the control of interpolated motion for machine tools, industrial robots, and motion stages using a network architecture.
Motion control systems are well known in the art. They enable much of the automated manufacturing that exists today, especially for machine tools, industrial robots, and xe2x80x9cPick and Placexe2x80x9d material handling systems. In general, they allow the precise rotational control of a servo motor which is coupled to a mechanical device that translates rotary motion into linear movement to facilitate the positioning of a work tool, another device, or product.
It is common for these types of systems to require interpolated motion, or coordinated and simultaneous motion of a plurality of servo motors (axes) which work together to move a tool, device, or product along a path defined within a three dimensional coordinate system.
The conventional state of the art system includes a central motion controller card which usually plugs into a computer bus adapter and processes a proprietary command set. Machine tool motion controllers, also known as Computer Numerical Controllers (CNC), are specialty computers with internal motion controller cards which process the RS-274D command set. There are also non-computer based multi-axis motion controllers, but, in general, they do not readily lend themselves to interpolated motion control at the level required for sophisticated machine tool processes.
Additional components of a motion control system typically include sensors, servo drives, servo motors, and encoders. The integration of these components with the central controller may easily require hundreds of feet of wire/cable and have hundreds of connection points. These large amounts of wire and numerous connection points increase the integration cost and decrease the reliability of a conventional motion control system.
To offer improved integration costs and system reliability, many controller manufacturers are providing alternative systems which distribute control over a network. The three dominate network protocols which support interpolated motion control are Sercos (IEC-61491), MACRO (Motion And Control Ring Optical), and Firewire (IEEE 1394). They are able to support interpolation because they are high bandwidth networks (up to 10 Mbit/sec) and implement deterministic network protocol. These require costly controller boards and specialized electronics within the servo drives. They do simplify integration and increase reliability, but suffer from increased component costs over the conventional approach. Additionally, the controllers are not compatible with the vast installed base of servo drives, so they do not provide an upgrade path, other then full system replacement.
There is a serial network protocol incorporated within all computers commonly known to as RS-232. This is a much slower network then those listed above and non-deterministic. When the serial output is distributed using the common RS-485 (multi-drop) parallel connection method, a bandwidth of approximately 115,200 bits/sec can be achieved. Therefore it has been used for utility or configuration purposes and non-interpolated motion sequencing. Accordingly, it would be desirable to provide a system and methodology which would enable this freely available serial network to be utilized for interpolated motion control. It would further be desirable that this system and methodology to be compatible with the existing installed base of machine tool servo drives and the low cost, commercially available, servo drive products having the standard analog signal control option.
U.S. Pat. No. Re. 30,132 (Irie) describes how a path to be followed by the point of a tool may be described as line segments specified by beginning and ending points. These beginning and ending points are input to a controller which interpolates all of the intermediate points on a real time basis and instructs the servo motors such that the point of the tool is commanded to pass though each of the intermediate points. An article by C. Wilson, xe2x80x9cHow Close Do You Have to Specify Points in a Contouring Application?xe2x80x9d, Motion Control, May 1993, relates how the modern controllers of today have become much more sophisticated since Irie. As described by Wilson, it is desirable to achieve more carefully tailored control of the motion of the tool then is possible by a single electronic circuit controller. To accommodate both the high travel rates and the ability to process typically 2,000 points per inch, a second circuit controller is used. The first acts as a course interpolation points generator and the second acts to add additional, or finer, points in between those points, the number of which is subject to the real time constraints of the process. Accordingly, it would be desirable to provide a system and methodology which would allow for xe2x80x9clook aheadxe2x80x9d and preprocessing of a portion of the path points by software and avoid the constraints of a real time system and the added cost of the resultant control electronics.
As shown in FIG. 6, the present invention controls a machine tool directly from the computer without add in cards by using a Control Module for each axis. The Control Module is attached near, or onto, each servo drive in the motion control system. Alternately, an embodiment of the invention can be incorporated within the servo drive as shown in FIG. 7 or within an integrated servo motor as shown in FIG. 8.
The Control Module may be developed using Digital Signal Processor (DSP) hardware. It is designed to communicate interactively with a computer via an RS232/RS485 network link to receive a control set point data stream. This data stream is generated by a Master Control program which resides in the computer""s memory. The DSP, or alternative processor type, is equipped to receive quadrature encoder feedback data which is compared to the set point data stream. Using a common PID based algorithm, it continuously updates an analog output signal which is able to control almost any commercially available servo drive. An optional feature includes a further aspect: A voltage controlled oscillator coupled to the analog output could interface the Control Module to step motor drives which require a step and direction pulse stream to control motion.
A set point buffer within each Control Module accumulates a backlog of control set points in a queue which enables the invention to operate within a non-deterministic operating system environment, like Windows. The computer serial port feeds data to the buffer on an irregular, but continuous basis and the Control Module provides the necessary real time functionality to control the servo drive.
With this system it is possible to process any length file because the path can be converted to set point data while the previous set point commands are being acted upon. Conversely, it is possible to pre-process a file and then send data out to the buffer as it is required to support motion.
To accomplish these features, a Control Module interfaces each axis"" servo drive to the computer via the RS-232 serial port which is connected using an RS-485 (multi-drop) network configuration. Each module has a unique address and ignores any information not having its"" identifier appended to it. The Master Control Program resides in computer memory and reads in the file containing the path information selected by the operator. This file can be a standard RS-274D CNC program file, DXF drawing file, HPGL plotter file, operator generated points list, or alpha numeric ASCII text commands. The path geometry is translated by a Master Control Program function into coordinate positions, or set points, having a spacing dependant upon an operator specified chord height tolerance (path departure maximum). Elapsed time values are calculated and added to each set point command line to provide the Control Module with the information necessary for it to calculate the required velocity and acceleration to apply to the servo motor. The set point command line string is transmitted through the serial port to each axis sequentially.
To begin the actual motion in the synchronized manner required for interpolation, and thereby compensate for the non-determinism of the RS-485 network, a digital start/synchronization circuit is used. The Control Module specified as the master has a high speed digital output connected to the inputs on each of the other Control Modules in the network. When the operator initiates a start command, after at least two set points have been sent to the Control Modules"" buffers, the start/synchronization circuit is strobed (on/off transition) by the master, causing all the axes to begin in precise unison. Any programmatic pauses in motion allow the individual Control Module internal clocks to be resynchronized with the master, again using the start/synchronization circuit. An embodiment of this invention could also maintain continuous path synchronization for an extended path length (or time) even if very inexpensive clocking electronics are utilized (which are more subject to non-determinate time drift) by strobing the start/synchronization circuit while motion is in process to re-synchronize the slaves"" internal clocks to the masters"" internal clock.
Another feature of the invention is that it provides a method in which existing motion control systems and machine tools can be retro fitted with the a network control methodology without the costly requirement of having to also replace the servo drives, as would be required if using the existing embodiments of the art. The savings can approach $8000 in a typical system.